stolen iPhone check

You can check if it’s been activated—>Check Your Service and Support Coverage - Apple Support

There is a legitimate site to check the IMEI (there are also a lot of scam sites, but this one is real)—>CTIA Stolen Phone Checker

This one is not 100%, as it only includes phones sold in the USA and only phones reported by carriers as stolen.

There probably is; look for a similar organization. CTIA is the association of cellular network providers in the US. Or call your carrier and ask them. You can also ask them if they can provision that specific phone on their network.

Use the first link I provided—>Check Your Service and Support Coverage - Apple Support. If it doesn’t appear here it has not been purchased by anyone, which means, if it is on the open market, it almost certainly stolen.

No.

Apple does not get involved with stolen devices. Why do you think it might be stolen? Only buy a phone from someone you trust or from an Apple Store.